Chemistry Reference
In-Depth Information
The spinning method for obtaining of amorphous Bi was used. Bi of
99.9999% purity was used as the source material. Before using this Bi charge, it
was purified by drop cleaning in vacuum, followed by directional crystallization.
After crystallization, a Bi polycrystal was obtained in pieces smaller than 7 g.
These pieces were placed into a quartz ampoule with 8-20 mm inner diameter and
115 mm length, which had an external resistance heater and an outer thermal
ceramic insulator. High purity Ar at a gauge pressure of 0.1 atm was passed
through the ampoule to prevent Bi melt oxidization. The melt temperature during
spinning was controlled from 750 K to 800 K. After the Bi melted, and a steady
state established, the melt was poured out onto a cold copper plate with 300 mm
diameter, rotating at 1100 rpm. Amorphous Bi formed as a film with 5-30 μm
thickness, 2-3 mm width, and 15-35 mm length. The Bi samples were annealed at
temperatures 150-220 °C for 1 h in a special cylindrical furnace with less than 1
K/cm temperature gradient in the axial and radial directions. A crossection of the
films was prepared by breaking the samples in liquid nitrogen. The film
microstructure was studied using a scanning electron microscope “Quanta 200 3D”.
The Bi films had a few microcrystals with sizes 10-30 μm after spinning
process, with thickness 20-30 μm. The amorphous Bi films had thickness near 5
μm. Nanocrystals were detected in amorphous Bi films only after annealing at
temperature above 150 °C. Individual nanocrystals with dimensions from 10 to
50 nm were sparsely located in a central part of the films after annealing at
150°C. Increasing the annealing temperature above 200 °C led to an increased
amount of nanocrystals with 50 - 100 nm dimensions and growth of crystallites
with sizes up to 500 nm. For these growth conditions, Bi crystallites had an
orientation of the (111) plane perpendicularly to the film surface.
Introduction
1.1. Crystal Structures
Bismuth (Bi) is a semimetal, and has a rhombohedral primitive unit cell
containing two atoms (Figure 1) [1]. Principal crystallographic axes of Bi are the
following: the trigonal axis of three-fold symmetry, the binary axes of two-fold
symmetry, and the bisectrix axis, which is perpendicular to the former two axes [2].
The bismuth structure may be regarded as a slightly distorted form of a simple
cubic lattice having superposition of two interpenetrating face-centered sublattices.
Bi crystals have double layer structure, which is shown in Figure 2 [3]. Within a
layer, the atoms are covalently bonded with some metallic bonding [4]. Between the
layers, there act metallic and van der Waals binding forces, so that the first
coordination atoms with smaller distance are placed within the layers. On other
hand, the next coordination sphere consists of atoms of the adjoining double layer.
